1. Field of the Invention
This application relates generally to primer compositions and articles incorporating the primer and, in one particular non-limiting embodiment, to a primer composition particularly useful for application onto polymeric substrates.
2. Technical Considerations
In the aviation industry, it is known to apply a microwave-absorbing material onto a glass aircraft transparency to help shield the aircraft instrumentation and the pilot from potentially harmful microwave radiation entering the aircraft. In one process, a microwave radiation-absorbing coating is applied onto a glass aircraft transparency substrate at elevated temperatures. This known process works well when coating glass substrates. However, more and more aircraft are being fitted with polymeric transparencies rather than glass transparencies to try to reduce the overall weight of the aircraft. When this known process of coating at elevated temperatures is practiced on polymeric substrates, as the polymeric substrate shrinks upon cooling, a compressive force is applied to the radiation-absorbing coating due to the increased thermal expansion and contraction forces associated with polymeric substrates as compared to glass substrates. This compressive force can cause the radiation-absorbing coating to either crack or pop off of the substrate as the substrate shrinks during cooling. Therefore, it is known to apply a primer layer onto the polymeric substrate before application of the radiation-absorbing coating. The primer layer acts as a buffer and helps protect the radiation-absorbing coating from the compressive forces caused by shrinkage of the plastic substrate.
However, problems still exist with the current process. For example, the primer layer must not only be strongly bonded to the underlying plastic substrate but must also bond strongly to the radiation-absorbing coating applied thereon. Additionally, it would be advantageous to decrease the resistance (ohms per square) of the radiation-absorbing coating to increase the protective properties of the coating. However, decreasing the resistance generally requires increasing the thickness of the coating. For example, if a particular coating provides a resistance of 20 ohms per square at a thickness of 3,500 Å, to decrease the resistance to 10 ohms per square typically requires doubling the thickness of the coating to 7,000 Å. Known primers are currently not capable of adhering a 10 ohm per square coating onto a plastic substrate using an elevated heating deposition process because the coating and/or primer delaminates from the substrate. In addition, the thicker the coating the more compressive stress that is applied as the polymeric substrate cools and, therefore, the more difficult it is to adhere the coating onto the substrate.
Therefore, it would be desirable to provide a primer that overcomes or reduces at least some of the problems discussed above with respect to known primers.